1. Field of the Invention
The invention relates to a method for operating a device for providing a liquid additive and to a corresponding device for carrying out the method according to the invention.
2. Related Art
Devices for providing a liquid additive are used in particular in the automotive field for supplying a liquid additive to the exhaust-gas treatment device of a motor vehicle. Of particularly great significance are exhaust-gas treatment devices in which nitrogen oxide compounds in the exhaust gas of an internal combustion engine are reduced with the aid of a reducing agent to form non-hazardous substances such as nitrogen, C02 and water. The corresponding exhaust-gas method is referred to as the SCR [Selective Catalytic Reduction] method.
Ammonia is normally used as reducing agent for this method. Ammonia is normally stored in motor vehicles not directly but rather in the form of a reducing agent precursor solution which can be stored as liquid additive and also provided to the exhaust-gas treatment device as liquid additive. The liquid additive is then converted, thermally or hydrolytically, within the exhaust gas to form the actual reducing agent ammonia. “Hydrolytically” means that the conversion reaction is assisted by a hydrolysis catalyst which may, for example, be provided on a substrate honeycomb body in the exhaust-gas treatment device. For the SCR method, a urea-water solution with a urea content of 32.5% is available under the trade name AdBlue®.
A problem of such liquid additives is that they can freeze at low temperatures. The described 32.5% urea-water solution, for example, freezes at −11° C. In the automotive field, such low temperatures can arise in particular during long standstill periods in winter. To be able to operate a device for providing liquid additive even at low temperatures despite the risk of freezing, and to make it possible in particular for such a device to be set in operation quickly after a long standstill phase, it is known for the device to be at least partially evacuated after a stoppage of operation. It can thus be ensured that no liquid additive is present at least in parts of the device, and thus freezing cannot occur there. It is further known for heaters to be provided in the devices for providing liquid additive, by which heaters at least parts of the device can be heated. By the heating, freezing of the liquid additive can be prevented, or frozen liquid additive can be thawed again.
Despite those measures, it is normally not possible for the device to be set in operation quickly if the liquid additive in a tank and within a delivery unit for delivering the liquid additive has completely frozen. Rather, it is necessary for the liquid additive in the tank to be at least partially melted. The liquid additive in the delivery unit must normally be completely thawed, or the delivery unit has been evacuated before the freezing process. The thawing of the frozen additive requires a certain period of time because, for the thawing of the corresponding volumes of liquid additive, a certain minimum amount of energy is required, which can be provided only at a limited rate.